1. Field of the Invention
The present invention relates generally to anisotropic melt-forming polymers (i.e., "liquid crystalline polymers" or simply "LCPs") and, more particularly, to blends of LCPs suitable for injection molding thin-walled molded articles, e.g., electrical connectors, having excellent weld-line strength and reduced warpage.
As an article of manufacture, the present invention is directed to molded articles having improved weld line strength, wherein the articles are fabricated from a polymer blend composition comprising at least one liquid crystalline polymer and at least one additional polymer, wherein the composition has a crystallization rate lower than said at least one liquid crystalline polymer.
The invention is based on our discovery that a given LCP composition which otherwise provides only marginal weld line strength can be markedly improved with respect to this property, by adding a second polymer, preferably another LCP, to obtain a blend which has a lower crystallization rate than at least one, and preferably each, of the LCP components of the blend. Such a blend has also been found to provide a reduction in warpage in the molded part.
As a composition of matter, the invention is directed to certain polymeric blends suitable for use in fabricating the molded articles of the invention. The blends comprise a first component comprising repeating units derived from terephthalic acid, isophthalic acid, p-hydroxybenzoic acid, hydroquinone and an arylene diol other than hydroquinone, (e.g., biphenol), in which the relative molar amounts of the units are such that the first component polyester falls approximately within the bounded regions shown in the triangular diagram of the accompanying FIG. 1.; and a second component, different from the first, comprising a wholly aromatic liquid crystalline polyester. Polyesters represented by the lower bounded region of FIG. 1 are particularly preferred as the first component in the blends of the present invention. A particularly preferred second component polymer of the blends is a copolyester comprising units derived from terephthalic acid, isophthalic acid, p-hydroxybenzoic acid and biphenol, wherein the relative molar amounts of the units are such that the composition falls approximately within the outer bounded region shown in the triangular diagram of FIG. 2. Polyesters within the inner bounded region are particularly preferred. Also preferred as the second component are copolyesters wherein at least one repeating ester unit comprises a naphthalene moiety.
The present invention is particularly advantageous for molding thin-walled electrical connectors which in the past have shown a tendency to break along the connector weld lines under typical conditions of assembly (i.e. pin insertion) where the connector must be incorporated into or assembled with other workpieces.
2. Background Discussion
Polymers capable of forming an anisotropic melt are well-known in the art, and are generally termed "liquid crystalline polymers" or "LCPs". The term "anisotropic" results from the finding, well documented in the literature, that the rod-like molecular chains characteristic of LCPs have a tendency in the molten or dissolved state to become unidirectionally aligned (i.e., "oriented") in parallel fashion. Evidence of this type of ordered structure includes the finding in the art that these materials are able to transmit light in optical systems equipped with crossed polarizers, whereas transmission of light is theoretically zero for isotropic materials. This distinguishing optical property, as well as a test for determining its existence in polymers, is discussed in numerous patents and publications. For example, reference may be had to the paper of R. Demartino entitled "Improved Processing of Thermotropic Liquid Crystalline Polyesters" which appeared in the Journal of Applied Polymer Science, Volume 28, pp. 1805-1810 (1983). Examples from the patent literature where the optical properties of LCPs and the measurement thereof have been discussed include Luise U.S. Pat. No. 4,274,514, at column 7, line 66 to column 10 line 26; and Maier et al. U.S. Pat. No. 5,015,689, at column 3, line 36 to column 4, line 2.
The tendency of LCPs toward anisotropic behavior in the melt distinguishes these polymers from the isotropic (disordered) behavior of most other molten polymers. A general discussion of thermotropic liquid crystalline polymers (the term "thermotropic" refers to LCPs which are ordered in the molten phase) can be found in the paper of H. Lingau et al. entitled "Thermotropic Liquid Crystal Polymers" published in Kunstoffe German Plastics 79 (1989) 10, pp. 81-83; and the paper of W. J. Jackson entitled "Liquid Crystal Polymers. XI. Liquid Crystal Aromatic Polyesters: Early History and Future Trends" published in Mol. Cryst. Liq. Cryst., 1989, Vol. 169, pp. 23-49.
The characteristic orientation of the LCP molecular chains in the molten state is preserved when the LCPs are processed into molded articles, fibers or films. Hence, the resulting products generally exhibit superior mechanical properties over conventional folded chain polymers. LCPs in which all of the repeating units comprise an aromatic ring (substituted or unsubstituted), i.e.: ##STR1## are termed wholly aromatic and have become well recognized in the art for their high temperature properties and toughness.
Another known benefit of injection moldable LCP polymers is the fact that their anisotropic behavior results in polymer melts having low viscosities compared to isotropic melt-forming polymers. The combination of high temperature performance, toughness, and the ability to form a relatively low viscosity melt, makes LCPs, and in particular the wholly aromatic LCPs, uniquely well suited for injection molding of thin-walled articles, such as electrical connectors, which must be strong but also capable of withstanding the temperature extremes encountered in vapor phase and infra-red soldering of electrical components. Some molded electrical components have fine structural details where the component may have wall thicknesses of not greater than about 0.03 inches. Thin-walled parts of this type would otherwise be very difficult to fill in the mold were it not for low viscosity polymers such as the commercially available LCPs. Examples of such LCPs are the oxybenzoyl copolyesters sold by Amoco Performance Products, Inc. under the tradename Xydar.RTM. and those sold by Hoechst-Celanese under the tradename Vectra.RTM.. Among the many patents which exemplify oxybenzoyl LCPs are Cottis et al U.S. Pat. Nos. 3,637,595 and 3,975,487; and Calundann U.S. Pat. No. 4,160,470. The term "oxybenzoyl" means that the LCP has a recurring unit which is derived from p-hydroxybenzoic acid: ##STR2##
Although LCPs such as the wholly aromatic oxybenzoyl copolyesters are uniquely suited to the molding of thin-walled electrical connectors, a well-known problem that has often been encountered in such molding is the tendency for the molded connector to exhibit a reduction in strength at any point where two or more molten resin flow fronts meet or join following injection of the resin into the mold. Such meeting points can be present if the mold contains one or more centrally located structure-defining barriers around which the injected resin must flow to fill the mold. They can also occur if resin is injected into the mold from more than one point of entry or gate. The boundary region where separate resin fronts flow together in the mold is commonly termed a "weld line" or a "knit-line". Unfortunately, it is generally known in the art that weld line quality in LCPs can be poorer than that of conventional thermoplastics. See, e.g., H. Bangert "Mouldings Produced from Liquid Crystal Polymers" Kunstoffe 79 (1989) 12, pp. 1327-1333. Hence, in the finished part, weld lines have been found to be areas of reduced strength where, under the stresses applied in manufacturing or use, the molded part may be more likely to undergo failure. Although weld line strength can in some instances be improved by adjusting the mold design, the position of the injection points, and other processing parameters, these ad hoc measures can be costly, inconvenient and generally must be tailored to each individual product. Therefore, need exists in the art for LCP compositions which exhibit an improvement in weld-line strength without regard to mold design or processing parameters.
Another problem encountered in the use of LCPs for injection molding of thin-walled articles is the occasional tendency for the molded part to display an unacceptable degree of warpage upon cooling after ejection from the mold. Although the problem of warpage can be controlled to some degree with appropriate fillers or through mold design, there remains a need for LCP compositions which exhibit better resistance to warpage.
A still further area where improvement in LCP compositions is sought is with respect to certain demanding applications where the injection molded part must undergo severe stresses during the manufacture of an article incorporating the molded part. In such instances it is desired to improve the ductility or toughness of LCPs, as measured in terms of properties such as tensile elongation. Manufacturers may frequently refer to "latch strength" when indicating the ability of a molded part to resist breakage during the assembly of an electrical product. Improvements in ductility are desired for improving the latch strength of electrical parts.
In view of the foregoing discussion, a general object of the present invention is to provide an improved LCP composition and molded articles therefrom. A further object is to provide an LCP composition which exhibits improved weld line strength. Still another object is to provide an LCP composition which exhibits a reduction in warpage. Other objects will become apparent hereinafter to those skilled in the art.